Gas turbine bucket with impingement cooled platform

ABSTRACT

In a turbine bucket having an airfoil portion and a root portion, with a substantially planar platform at an interface between the airfoil portion and root portion, a platform cooling arrangement including at least one bore in the root portion and at least one impingement cooling tube seated in the bore, the tube extending beyond the bore with an outlet in close proximity to a targeted area on an underside of the platform.

This application is a continuation of Ser. No. 09/404,589 filed Sep. 24,1999.

This invention was made with Government support under Contract No.DE-FC21-95MC31176 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

This invention relates to the cooling of turbine buckets and,specifically, to the cooling of the platform region of the bucket, atthe base of the airfoil.

BRIEF SUMMARY OF THE INVENTION

Stage one and stage two buckets in gas turbines experience hightemperatures and stress, creating low cycle fatigue life concerns,particularly at the leading edge fillet where the airfoil joins theplatform at the forward portion of the pressure side of the airfoil.

In aircraft engines, the bucket platforms are cooled by air entering thecavity below the platform, but this arrangement is not completelysatisfactory, particularly in connection with the leading edge fillet.

This invention provides a unique solution to the above problem bydirecting cooling air through a tube, the forward end of which islocated closely adjacent the underside of the platform, below theforward portion of the pressure side of the airfoil, so as toeffectively impingement cool the platform in the localized region belowthe leading edge fillet.

In addition, film cooling holes are formed in the platform itself sothat air introduced into the cavity by the impingement cooling tube canescape the cavity and flow along the upper surface of the platform alongthe contour of the pressure side of the airfoil, in a direction towardthe trailing edge of the airfoil.

In its broader aspects, the invention relates to a turbine bucket havingan airfoil portion and a root portion, with a substantially planarplatform at an interface between the airfoil portion and root portion, aplatform cooling arrangement comprising at least one bore in the rootportion and at least one impingement cooling tube seated in the bore,the tube extending beyond the bore with an outlet in close proximity toa targeted area on an underside of the platform.

In another aspect, the invention relates to a method of cooling aleading edge fillet region of a turbine bucket having an airfoil portionand a root portion, the airfoil portion joined to a platform extendingover the root portion, comprising: a) providing a through bore in aleading side of a cover plate in the root portion, the borecommunicating with a recess in the root portion underlying the airfoilportion; b) inserting a tube in the bore, with a tip of the tubeextending beyond the bore and in close proximity to a targeted area onan underside of the platform; c) supplying cooling air to the recess viathe tube such that the targeted area is impingement cooled by thecooling air passing through the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side cross section of a bucket in accordance withthe invention;

FIG. 2 is a partial end view of a lower dovetail portion (of the leadingside edge) of the bucket not entirely shown in FIG. 1; and

FIG. 3 is a top plan view of the bucket in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1-3, a turbine bucket 10 includes an airfoilportion 12 extending vertically upwardly from a horizontal,substantially planar platform 14. The airfoil portion has a leading edge15 and a trailing edge 17. Below the platform 14, there are two pair ofso-called “angel wings” 16, 18 extending in opposite directions from theleading and trailing sides 20, 22 of the cover plate 24 in the rootportion of the bucket. The platform 14 is joined with the cover plate24. Below the root portion, there is a dovetail 26 by which the bucketis secured to a turbine wheel (in a preferred embodiment, the stage 1 orstage 2 wheels of a gas turbine).

Cooling passages (not shown) extend in radially outward and inwarddirections in a serpentine path through the airfoil portion, the pathhaving passages in the root portion communicating with supply and returnpassages 28, 30 in the dovetail. The internal airfoil cooling circuitforms no part of the invention, however, and is therefore not shown indetail. Of significance here is the substantially hollow space or recess32 in the root portion, above the dovetail 26 but below the platform 14,defined further by the sides 20, 22 of the cover plate. This recess isgenerally on the pressure side 34 of the airfoil, and it will beappreciated from FIG. 3 that a relatively large portion of the platform14 on the pressure side 34 of the airfoil overlies the hollow space orrecess 32.

An angled hole 36 is drilled through the leading side 20 of the coverplate 24 from the lower end of the root portion, on the external sidethereof, extending upwardly (i.e., substantially radially) and openinginto the recess 32. Due to the angled orientation, the hole 36 has anexit profile on the interior of the root wall 38 which is elliptical inshape as shown in FIG. 2. An impingement cooling tube 40 is pressed intothe hole 36 from under the root portion, so that an outlet at the upperend or tip 42 of the tube is located close to the underside of theplatform, and close to a targeted area along the leading edge fillet 44where the airfoil joins the platform.

The lower end of the tube 40 is flared at 46 to limit the extent ofinsertion, and the tube may be fixed in place by a spot weld at 47 toprevent the tube from sliding back through the hole in a directionopposite the assembly direction. Preferably, the tube end 42 is withinabout 0.5″ from the underside of the platform.

Since the lower edge of the cover plate 24 will seat on the wheelsurface, thus blocking the hole 36, a second hole 48 is drilled from theleading side 20 of the cover plate 24 so as to intersect with the hole36, just below the lower end of the tube 40. In this way, rotor or wheelspace purge air can enter the tube 40 via hole 36.

In addition, an array of film cooling holes 50 extend through theplatform 14 from the recess 32 to a location on the external side of theplatform.

With the above arrangement, purge air from wheel space below the bucketwill enter the hole 48 and flow through the tube 40 so as to impingedirectly on the underside of the platform 14, below but proximate theleading edge fillet 44. The underside of the platform 14 is thusimpingement cooled in the leading edge fillet area to reduce stress andimprove low cycle fatigue margins. Of course, the enhancement of heattransfer in this specific region continues along the platform and atleast partially under the airfoil itself.

Another feature of the invention lies in the combination of the abovedescribed impingement cooling with film cooling along the upper surfaceof the platform: This is achieved by utilizing the array of film coolingholes 50 in the platform, allowing the cooling air in the hollow space32 to exit along the top surface of the platform. The film cooling holes50 are directionally oriented so that the cooling air merges with thegas path air and flows along the pressure side of the airfoil toward thetrailing edge 17. The flowpath of the film cooling air is such that itcan carry over to the suction side of the platform of the adjacentbucket, increasing the overall effectiveness of the system.

To further enhance heat transfer, the underside of the platform 14 canbe provided with any suitable rough coat, thus increasing the surfacearea available for heat transfer. In addition, one or more additionalimpingement cooling tubes may be used in conjunction with the tube 40 toenhance heat transfer at targeted locations along the underside of theplatform.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. In a turbine bucket having an airfoil portion anda root portion, with a substantially planar platform at an interfacebetween the airfoil portion and root portion, a platform coolingarrangement comprising: at least one bore in said root portion and atleast one impingement cooling tube seated in said at least one bore,said tube extending beyond said bore with an outlet in close proximityto a targeted area on an underside of said platform.
 2. The bucket ofclaim 1 wherein said targeted area lies within a recess under saidplatform, and proximate a leading edge fillet of said airfoil portion.3. The bucket of claim 2 wherein said platform is provided with aplurality of film cooling holes communicating with said recess.
 4. Thebucket of claim 3 wherein said plurality of film cooling holes arelocated closer to a trailing edge of said airfoil portion than to theleading edge of said airfoil portion.
 5. The bucket of claim 4 whereinsaid impingement cooling tube and said plurality of film cooling holesare located on a pressure side of said airfoil portion.
 6. The bucket ofclaim 3 wherein said film cooling holes are directionally oriented tocause cooling air to flow in a path generally toward said trailing edgeof said airfoil portion.
 7. The bucket of claim 1 wherein the undersideof said platform is coated with a rough coat material.
 8. The bucket ofclaim 1 wherein said impingement cooling tube is flared at its rearwardend to limit forward insertion into said bore.
 9. The bucket of claim 1wherein said impingement cooling tube is secured within said bore so asto prevent movement in a direction away from said platform.
 10. In aturbine bucket having an airfoil portion and a root portion, with asubstantially planar platform at an interface between the airfoilportion and root portion, a platform cooling arrangement comprising:means for impingement cooling a targeted area on an underside of saidplatform.
 11. A method of cooling a leading edge fillet region of aturbine bucket having an airfoil portion and a root portion, saidairfoil portion joined to a platform extending over said root portion,comprising: a) providing a through bore in a leading side of a coverplate in said root portion, said bore communicating with a recess insaid root portion underlying said airfoil portion; b) inserting a tubein said bore, with a tip of the tube extending beyond said bore and inclose proximity to a targeted area on an underside of said platform; c)supplying cooling air to said recess via said tube such that saidtargeted area is impingement cooled by said cooling air passing throughsaid tube.
 12. The method of claim 11 wherein said targeted area is anarea in said recess substantially directly beneath the leading edgefillet of said airfoil portion.
 13. The method of claim 11 includingproviding film cooling holes to allow said cooling air to escape fromsaid recess.
 14. The method of claim 11 wherein said film cooling holesare located in said platform.
 15. The method of claim 14 wherein saidimpingement cooling tube and said film cooling holes are on a pressureside of said airfoil portion.
 16. The method of claim 15 wherein saidfilm cooling holes are directionally oriented to cause cooling air toflow in a path generally toward said trailing edge of said airfoilportion.